vstd: add specs for BinaryHeap, BTreeMap, and BTreeSet

[briangmilnes]

Add formal specifications for three Rust stdlib collection types (easy additions suggested by veracity-analyze_rust_wrapping_needs).

• BinaryHeap: push, pop, peek, len, is_empty, clear, with DeepView support and is_heap_max maximality predicate
• BTreeMap: new, insert, get, contains_key, remove, clear, clone, len, is_empty, with iterators (Keys, Values, Iter, IntoIter) and DeepView
• BTreeSet: new, insert, remove, contains, len, is_empty, clear, clone, with iterators (Iter, IntoIter) and DeepView

Includes comprehensive tests covering:

• All methods with value verification
• Iterator tests with for-loop, while-loop, and loop-match patterns
• DeepView tests for nested types

Also adds CONTRIBUTING.md section on controlling test parallelism (RUST_TEST_THREADS) since vargo does not forward -j flag.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[tjhance]

I haven't looked at this in detail, but at the very least, these containers need to use some sort of obeys_key_model assumption (similar to our specs for HashMap), i.e., an assumption that comparisons are deterministic and that equality is compatible with Ord.

[briangmilnes]

10q! Got it, but this is going to take some work and I might as well then put in my additions for Ordering in cmp. Which I trust more now. What's the process? Do I just close out this PR as it's getting dated? Get up to date and start a new one?